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>  News Releases >   2007 >   July

Dartmouth Organic Farm builds "greener" greenhouse

Dartmouth College Office of Public Affairs • Press Release
Posted 07/06/07 • Rebecca Bailey • (603) 646-3661

When it comes to energy use, conventional greenhouses aren't all that "green." For example, it takes hundreds of gallons of propane each year to heat the 816-square-foot greenhouse at the Dartmouth Organic Farm.

Not so for the farm's second greenhouse, being built new this summer on the farm's site on Route 10, three miles north of campus: thanks to passive-solar and energy-conserving features, this one won't need fossil fuel to provide a reasonable growing environment.

Site work, foundation, and construction of the greenhouse's frame and glazing is expected to start in July and conclude by fall in time to start growing cool-weather plants. As with other farm operations, a substantial amount of the work will be done by student volunteers. Meanwhile, funds are being sought to complete the final phases, including the features that should make the greenhouse able to sustain temperatures above freezing most of the year with only the sun's heat.

From L-R, Outdoors Program Director Andy Harvard, Thayer School students and greenhouse designers Luke Wachter and Chris Polashenski, and Farm Manager Scott Stokoe, with the farm's propane-heated greenhouse in the background. (Photo by Joseph Mehling '69)

The design was the work of two students at Dartmouth's Thayer School of Engineering, with the help of White River Junction, VT-based architect-builder Daniel Johnson, who drew up the final plans and will oversee construction. The students, Chris Polashenski and Luke Wachter, chose the greenhouse as their project for Thayer School's 190/290 two-course sequence on product design, in which students choose a real problem posed by a real client and follow the standard product design methodology, meeting with their clients and researching, testing, and formally presenting design options.

The farm's challenge involved taking the disassembled frame and rigid plastic glazing of a conventional, 1960s-era Lord & Burnham greenhouse and turning it into a solar-only greenhouse. The farm was given the greenhouse by the U.S. Army Cold Regions Research and Engineering Laboratory in Hanover

Wachter said he was drawn to the project because it was a chance to do environmentally friendly work, would involve creating computer simulations, and would allow him to work with the college farm, something he'd had always wanted to do. He and Polashenski were awarded the Engineering Design Prize from Thayer School's Corporate Collaboration Council when they received their Bachelor of Engineering degrees this past June.

The students' phone-book-sized final report includes rigorous cost-benefit analyses of each retrofit option to make sure the improvements will pay for themselves in the amount of fuel they would save and how much more income they could provide through the crops they would enable a greenhouse to grow.

This cost-effectiveness was a critical part of the design for Farm Manager Scott Stokoe, who hopes the new greenhouse will offer a helpful model for growers who want to modify their greenhouses for greater energy efficiency. "I believe that there will be a general push for more locally grown out-of-season food production, in response to the issues of reducing carbon loads and the rising cost of fuel, coupled with the local foods movement. We wanted to model a way to transition away from fossil-fuel season extension to renewable season extension, but we wanted to do it so it wouldn't be out of reach of the typical grower," Stokoe said.

Many of the students' recommendations made it into the final design including:

  • Southern orientation and insulated north wall: Conventional greenhouses are usually sited with their long sides facing east and west and are clad entirely in translucent, uninsulated glazing, to allow the maximum amount of daylight to fall on the plants. Like other solar greenhouses, this one will have its long sides facing south and north, and the northern wall will be opaque and insulated. This allows the greenhouse to get almost as much daylight as a conventionally sited greenhouse but to lose much less heat.
  • Thermal mass: Conventional greenhouses devote as much space as possible to the plants themselves. This greenhouse will sacrifice some growing space to make room for "thermal mass": structures that absorb heat during the warmer portion of the 24-hour cycle and release it during the cooler portion, evening out the temperature spikes. In this greenhouse, thermal mass will be provided by 12 734-gallon cylindrical water tanks placed along the northern wall.
  • A night curtain: Once day is done, an insulated blanket will move into place in tracks, creating a thermal barrier between the plants and the greenhouse glazing to keep the heat inside the greenhouse from radiating out into the cool night air.

A view inside the 1/12 scale model Wachter and Polashenski made of the new greenhouse, with cups of antifreeze playing the role of the eventual 734-gallon thermal storage tanks. The students used the model to make measurements of the efficacy of greenhouse retrofits. (Photo courtesy Luke Wachter '06)

The 12 water tanks also provide a site for the project's most experimental aspect: sustainable aquaculture. This past year two Dartmouth undergraduates—Dunya Onen, who graduated in June, and Aurora Coon, who will be a sophomore this fall—began what is expected to be a long process of investigation to find edible, marketable aquatic species that can be raised in the tanks. Ideally, Stokoe said, the species would be native to the Northeast and the tanks would be "polycultures"—mimicking the kinds relationships of species found in a natural ecosystem.

Add to this bins of worms composting plant litter, and the greenhouse becomes almost a "closed loop," environmentally—with worms being one of the food sources for the fish in the aquaculture tanks, and waste from the tanks helping to fertilize the plants, which then provide the litter for the worms. While not a true closed loop, in which water and nutrients endlessly cycle with no outside input other than the sun's warmth and light, the greenhouse would at least provide an "educational model" of this key environmental concept Stokoe said.

Started in 1996, the Organic Farm is an educational and working agricultural enterprise run by students who grow, harvest and sell locally a wide variety of fruits, vegetables and flowers. Much of the labor is performed manually by farm club members on the scenic banks of the Connecticut River. Located on roughly 200 acres that the College owns on Route 10 three miles north of the campus , the farm includes a one-acre vegetable garden, a small sugar bush for gathering maple sap each spring, and a propane-heated greenhouse in addition to the one under construction.

The farm also serves as an outdoor classroom and laboratory for students studying botany, agriculture, and geography. Some 12 to 15 classes per year use the farm for some portion of their instruction. "In 11 years, the farm has grown from an organic farm with an important but limited mandate to what it is now, a campus center for environmental, ecological, and sustainability activity," said Andy Harvard, director of the Outdoor Programs Office, which oversees the farm.

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